https://raw.githubusercontent.com/elastic/elasticsearch/master/buildSrc/src/test/java/org/elasticsearch/gradle/ConcatFilesTaskTests.java
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 * ownership. Elasticsearch licenses this file to you under
 * the Apache License, Version 2.0 (the "License"); you may
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 * You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
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package org.elasticsearch.gradle;

import java.io.File;
import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.util.Arrays;

import org.elasticsearch.gradle.test.GradleUnitTestCase;
import org.gradle.api.Project;
import org.gradle.testfixtures.ProjectBuilder;

public class ConcatFilesTaskTests extends GradleUnitTestCase {

    public void testHeaderAdded() throws IOException {

        Project project = createProject();
        ConcatFilesTask concatFilesTask = createTask(project);

        concatFilesTask.setHeaderLine("Header");

        File file = new File(project.getProjectDir(), "src/main/java/Code.java");
        file.getParentFile().mkdirs();
        file.createNewFile();
        concatFilesTask.setTarget(file);
        concatFilesTask.setFiles(project.fileTree("tmp/"));

        concatFilesTask.concatFiles();

        assertEquals(Arrays.asList("Header"), Files.readAllLines(concatFilesTask.getTarget().toPath(), StandardCharsets.UTF_8));

        file.delete();
    }

    public void testConcatenationWithUnique() throws IOException {

        Project project = createProject();
        ConcatFilesTask concatFilesTask = createTask(project);

        File file = new File(project.getProjectDir(), "src/main/java/Code.java");
        file.getParentFile().mkdirs();
        file.createNewFile();
        concatFilesTask.setTarget(file);

        File file1 = new File(project.getProjectDir(), "src/main/input/java/file1.java");
        File file2 = new File(project.getProjectDir(), "src/main/input/text/file2.txt");
        file1.getParentFile().mkdirs();
        file2.getParentFile().mkdirs();
        file1.createNewFile();
        file2.createNewFile();
        Files.write(file1.toPath(), ("Hello" + System.lineSeparator() + "Hello").getBytes(StandardCharsets.UTF_8));
        Files.write(file2.toPath(), ("Hello" + System.lineSeparator() + "नमस्ते").getBytes(StandardCharsets.UTF_8));

        concatFilesTask.setFiles(project.fileTree(file1.getParentFile().getParentFile()));

        concatFilesTask.concatFiles();

        assertEquals(Arrays.asList("Hello", "नमस्ते"), Files.readAllLines(concatFilesTask.getTarget().toPath(), StandardCharsets.UTF_8));

    }

    private Project createProject() {
        Project project = ProjectBuilder.builder().build();
        return project;
    }

    private ConcatFilesTask createTask(Project project) {
        return project.getTasks().create("concatFilesTask", ConcatFilesTask.class);
    }

}
